Theresa May is facing a fresh constitutional clash with the Scottish government after Nicola Sturgeon threatened to block the “great repeal bill”, dismissing it as a “naked power grab”.

The bill requires the consent of the Scottish parliament and the Welsh assembly, but Scotland’s first minister and Wales’s Carwyn Jones issued a joint statement saying they cannot recommend the bill as it stands to their respective legislatures.

Sturgeon has been careful to avoid describing the consent of the Scottish parliament as a veto. There is an issue as to whether Westminster could override the Scottish parliament.

The Scottish secretary, David Mundell, said Holyrood’s approval through a legislative consent motion would be required for the bill, but added: “I am optimistic we will obtain the consent.”

He suggested that the impasse would be resolved through negotiation with Sturgeon.

The Scottish National party leader is digesting her party’s losses in the general election last month and may decide it is not in its interest to block the bill, instead opting to use the leverage to negotiate the transfer of as many powers as possible from Brussels to Scotland, rather than Westminster.

Mundell denied that the transfer would turn out to be a power grab by Westminster and insisted that it would turn out to be a “power bonanza” for Scotland. Sturgeon would find it difficult to block a bill that would transfer powers from Brussels to Edinburgh, he added, but declined to specify which powers these may be, other than hill-farming.

During the journalists’ briefing, Mundell spoke vaguely about environmental and energy issues, avoiding contentious topics such as fishing rights.

“Needless to say there will be a process row with the Scottish government, because the Scottish government does process row, that is their speciality,” he said, adding that previous standoffs had been resolved.

In a joint statement, Sturgeon and Jones said: “We have repeatedly tried to engage with the UK government on these matters and have put forward constructive proposals about how we can deliver an outcome which will protect the interests of all the nations in the UK, safeguard our economies and respect devolution.

“Regrettably, the bill does not do this. Instead, it is a naked power grab, an attack on the founding principles of devolution and could destabilise our economies. The European Union (withdrawal) bill does not return powers from the EU to the devolved administrations as promised.

“It returns them solely to the UK government and parliament, and imposes new restrictions on the Scottish parliament and national assembly for Wales.

“On that basis, the Scottish and Welsh governments cannot recommend that legislative consent is given to the bill as it currently stands.”